<s>
In	O
relational	B-Algorithm
algebra	I-Algorithm
,	O
a	O
selection	O
(	O
sometimes	O
called	O
a	O
restriction	O
in	O
reference	O
to	O
E.F.	O
Codd	O
's	O
1970	O
paper	O
and	O
not	O
,	O
contrary	O
to	O
a	O
popular	O
belief	O
,	O
to	O
avoid	O
confusion	O
with	O
SQL	B-Language
's	O
use	O
of	O
SELECT	B-Language
,	O
since	O
Codd	O
's	O
article	O
predates	O
the	O
existence	O
of	O
SQL	B-Language
)	O
is	O
a	O
unary	O
operation	O
that	O
denotes	O
a	O
subset	O
of	O
a	O
relation	O
.	O
</s>
<s>
or	O
where	B-Language
:	O
</s>
<s>
The	O
selection	O
denotes	O
all	O
tuples	B-Application
in	O
for	O
which	O
holds	O
between	O
the	O
and	O
the	O
attribute	O
.	O
</s>
<s>
The	O
selection	O
denotes	O
all	O
tuples	B-Application
in	O
for	O
which	O
holds	O
between	O
the	O
attribute	O
and	O
the	O
value	O
.	O
</s>
<s>
For	O
an	O
example	O
,	O
consider	O
the	O
following	O
tables	O
where	B-Language
the	O
first	O
table	O
gives	O
the	O
relation	O
,	O
the	O
second	O
table	O
gives	O
the	O
result	O
of	O
and	O
the	O
third	O
table	O
gives	O
the	O
result	O
of	O
.	O
</s>
<s>
A	O
generalized	O
selection	O
is	O
a	O
unary	O
operation	O
written	O
as	O
where	B-Language
is	O
a	O
propositional	O
formula	O
that	O
consists	O
of	O
atoms	O
as	O
allowed	O
in	O
the	O
normal	O
selection	O
and	O
,	O
in	O
addition	O
,	O
the	O
logical	O
operators	O
(	O
and	O
)	O
,	O
(	O
or	O
)	O
and	O
(	O
negation	O
)	O
.	O
</s>
<s>
This	O
selection	O
selects	O
all	O
those	O
tuples	B-Application
in	O
for	O
which	O
holds	O
.	O
</s>
<s>
For	O
an	O
example	O
,	O
consider	O
the	O
following	O
tables	O
where	B-Language
the	O
first	O
table	O
gives	O
the	O
relation	O
and	O
the	O
second	O
the	O
result	O
of	O
.	O
</s>
<s>
In	O
SQL	B-Language
,	O
selections	O
are	O
performed	O
by	O
using	O
WHERE	B-Language
definitions	O
in	O
SELECT	B-Language
,	O
UPDATE	B-Language
,	O
and	O
DELETE	B-Language
statements	O
,	O
but	O
note	O
that	O
the	O
selection	O
condition	O
can	O
result	O
in	O
any	O
of	O
three	O
truth	O
values	O
(	O
true	O
,	O
false	O
and	O
unknown	O
)	O
instead	O
of	O
the	O
usual	O
two	O
.	O
</s>
<s>
In	O
SQL	B-Language
,	O
general	O
selections	O
are	O
performed	O
by	O
using	O
WHERE	B-Language
definitions	O
with	O
AND	O
,	O
OR	O
,	O
or	O
NOT	O
operands	O
in	O
SELECT	B-Language
,	O
UPDATE	B-Language
,	O
and	O
DELETE	B-Language
statements	O
.	O
</s>
